NHibernate DuplicateMappingException when mapping abstract class and subclass

Discovered the problem. It turned out that I did reference the same Assembly several times in the PersistenceModel used to configure the database:

public class MappingsPersistenceModel : PersistenceModel
{
    public MappingsPersistenceModel()
    {
        AddMappingsFromAssembly(typeof(FooMapping).Assembly);
        AddMappingsFromAssembly(typeof(BarMapping).Assembly);
        // Where FooMapping and BarMapping is in the same Assembly. 
    }
}

Apparently this is not a problem for ClassMap-mappings. But for SubclassMap it doesn't handle it as well, causing duplicate mappings - and hence the DuplicateMappingException. Removing the duplicates in the PersistenceModel fixes the problem.

If you are using automappings together with explicit mappings then fluent can generate two mappings for the same class.